1. Field of the Invention
The invention relates to an antenna impedance adjuster suitable for a portable radio-signal transceiver which transmits and receives radio signals in time division multiple access (TDMA) system.
2. Descriptuion of the Related Art
A portable radio-signal transceiver is presently designed to have a radio signal transmitting band and a radio signal receiving band for making communication through radio signals. In general, a radio signal transmitting band is designed to have a frequency band remote from the same of a radio signal receiving band.
A whip antenna usually has a relatively high impedance. The impedance is different from a characteristic impedance of radio-signal receiving and transmitting circuits, and hence, it is necessary to match impedance between the whip antenna and the radio-signal receiving and transmitting circuits in order to minimize transmission loss therebetween.
FIG. 1 illustrates a conventional portable radio-signal transceiver. As illustrated in FIG. 1, the portable radio-signal transceiver 1 is comprised of a whip antenna 2, a matching circuit 4 electrically connected to the whip antenna 2, a radio signal transmitting circuit 8, a radio signal receiving circuit 9, a switch 7 for electrically connecting the matching circuit 4 to one of the radio signal transmitting and receiving circuits 8 and 9, and a control circuit for controlling operation of the switch 7, the radio signal transmitting circuit 8, and the radio signal receiving circuit 9.
The portable radio-signal transceiver 1 operates as follows.
When a radio signal is to be transmitted, the control circuit 10 transmits a control signal S2 to the radio signal transmitting circuit 8 to thereby operate the radio signal transmitting circuit 8, and at the same time, transmits a control signal S1 to the switch 7 to thereby cause the switch 7 to electrically connect the matching circuit 4 to the radio signal transmitting circuit 8 therethrough. Thus, the radio signal transmitting circuit 8 transmits a radio signal through the matching circuit 4 and the whip antenna 2.
When a radio signal is to be received, the control circuit 10 transmits a control signal S3 to the radio signal receiving circuit 9 to thereby operate the radio signal receiving circuit 9, and at the same time, transmits a control signal S1 to the switch 7 to thereby cause the switch 7 to electrically connect the matching circuit 4 to the radio signal receiving circuit 9 therethrough. Thus, the radio signal receiving circuit 9 receives a radio signal through the matching circuit 4 and the whip antenna 2.
In the conventional portable radio-signal transceiver illustrated in FIG. 1, since the switch 7 is switched between the radio signal transmitting and receiving circuits 8 and 9 in accordance with the control signal S1 transmitted from the control circuit 10 for making radio signal communication through the matching circuit 4, the matching circuit 4 has to match impedance both in a radio signal transmitting band and a radio signal receiving band.
However, when the matching circuit 4 matches the antenna 2 to the radio signal transmitting and receiving circuits 8 and 9, it becomes more difficult to match all transmitting and receiving bands to characteristic impedance of the radio signal transmitting and receiving circuits 8 and 9 due to fluctuation in frequency characteristics of the antenna 2 and the matching circuit 4, as the radio signal transmitting and receiving bands become wider. As a result, the portable radio signal transceiver has to operate with transmission loss being generated to some degree.
In addition, the conventional portable radio signal transceiver is accompanied further with problems that an output transmitted from the radio signal transmitting circuit 8 varies due to an input impedance in a radio signal transmitting band of the antenna 2, and that a noise figure (NF) is deteriorated due to mismatching with an input impedance of the radio signal receiving circuit 9, resulting in degradation in sensitivity for receiving radio signals.
Japanese Unexamined Patent Publication No. 6-244756 has suggested an apparatus for matching antenna impedance. The suggested apparatus a first impedance matching circuit used when radio signals are to be transmitted, a second impedance matching circuit used when radio signals are to be received, a switch which electrically connects an antenna to one of the first and second impedance matching circuits, and a controller which receives timing signals of TDMA type to thereby control the switch.
Japanese Unexamined Patent Publication No. 8-274679 has suggested a portable telephone comprising an antenna, a matching circuit which is designed to switch matching characteristics, and a control circuit which transmits a control signal for switching matching characteristics of the matching circuit between radio signal transmission and radio signal receipt.
Japanese Unexamined Patent Publication No. 9-331206 has suggested an antenna matching circuit suitable for TDMA type portable telephone. In operation, a radio signal receiver receives radio signals from an antenna, and a radio signal transmitter transmits radio signals through the antenna. Radio signals to be transmitted have a frequency different from a frequency of radio signals to be received. A first antenna matching circuit matches impedance in the antenna at a frequency of received signals, and a second antenna matching circuit does the same at a frequency of transmitted signals. A controller electrically connects the first antenna matching circuit and the radio signal receiver to the antenna while a radio signal is being received, and electrically connects the second antenna matching circuit and the radio signal transmitter to the antenna while a radio signal is being transmitted.
Japanese Unexamined Patent Publication No. 6-244756 has suggested an antenna-impedance matching circuit comprising first means for matching impedance, which is electrically connected to an antenna, and has a switch for switching impedance between radio signal transmission and radio signal receiving to thereby match impedance to an antenna, and a controller which receives timing signals each indicating a timing at which radio signals are to be transmitted and received, to thereby control an operation of the switch so that impedance of the first means is matched to the antenna.
Japanese Unexamined Patent Publication No. 8-274679 has suggested a portable cellular phone making communication in time division multiple access (TDMA), comprising an antenna through which a radio signal is to be transmitted and received, a variable matching circuit which varies matching characteristics, and a control circuit which transmits a control signal by which matching characteristics of the matching circuit is switched between radio signal transmission and radio signal receipt.
In view of the above-mentioned problems in the conventional portable radio-signal transceiver, it is an object of the present invention to provide an antenna impedance adjuster which is capable of reducing transmission loss between an antenna and internal circuits to thereby enhance output-load characteristic of a radio signal transmitting circuit, and preventing degradation in sensitivity of a radio signal receiving circuit.
There is provided an antenna impedance adjuster including (a) an antenna through which radio signals are received and transmitted, (b) a radio signal receiver which receives radio signals having been received through the antenna, (c) a radio signal transmitter which transmits radio signals through the antenna, (d) an adjuster which matches an impedance between the antenna and the radio signal receiver in radio signal receiving band and an impedance between the antenna and the radio signal transmitter in radio signal transmitting band, (e) a first switch which electrically connects the adjuster to one of the radio signal receiver and the radio signal transmitter, and (f) a controller which controls an operation of the first switch when a radio signal is received or transmitted.
It is preferable that the adjuster is designed to have a variable circuit structure, and the controller varies a circuit structure of the adjuster when a radio signal is to be received or transmitted.
For instance, the adjuster may be designed to include (d1) a first adjuster which matches an impedance between the antenna and the radio signal receiver in radio signal receiving band, (d2) a second adjuster which matches an impedance between the antenna and the radio signal transmitter in radio signal transmitting band, when electrically connected to the first adjuster, and (d3) a second switch which disconnects the second adjuster from the first adjuster when a radio signal is to be received, and electrically connects the second adjuster to the first adjuster when a radio signal is to be transmitted.
It is preferable that the controller transmits a control signal to the second switch, and the second switch makes its operation in accordance with the control signal.
For instance, the first adjuster may be comprised of (d11) an inductance electrically connected in series to the antenna through one end thereof, and electrically connected to the second switch through the other end thereof, and (d12) a first capacitor electrically connected to the other end of the inductance at one end thereof, and grounded at the other end thereof, the second adjuster is comprised of a second capacitor electrically connected at one end thereof to a point at which the first capacitor is connected to the inductance, and grounded at the other end thereof, and the second switch is comprised of a switching device which turns the second capacitor off when a radio signal is to be received, and turns the second capacitor on when a radio signal is to be transmitted.
It is preferable that the antenna impedance adjuster further includes an on-off controller for controlling the switching device to turn on or off in accordance with a control signal transmitted from the controller.
As an alternative, the first adjuster may be comprised of (d11) a first inductance electrically connected in series to the antenna through one end thereof, and electrically connected to the second switch through the other end thereof, and (d12) a first capacitor electrically connected to the other end of the inductance at one end thereof, and grounded at the other end thereof, the second adjuster is comprised of a second inductance electrically connected at one end thereof to a point at which the first capacitor is connected to the first inductance, and grounded at the other end thereof, and the second switch is comprised of a switching device which turns the second inductance off when a radio signal is to be received, and turns the second inductance on when a radio signal is to be transmitted.
For instance, the first adjuster may be comprised of (d11) a first inductance electrically connected in series to the antenna through one end thereof, and electrically connected to the second switch through the other end thereof, and (d12) a second inductance electrically connected to the other end of the first inductance at one end thereof, and grounded at the other end thereof, the second adjuster is comprised of a third inductance electrically connected at one end thereof to a point at which the first inductance is connected to the second inductance, and grounded at the other end thereof, and the second switch is comprised of a switching device which turns the third inductance off when a radio signal is to be received, and turns the third inductance on when a radio signal is to be transmitted.
For instance, the first adjuster may be comprised of (d11) a first inductance electrically connected in series to the antenna through one end thereof, and electrically connected to the second switch through the other end thereof, (d12) a second inductance electrically connected to the other end of the first inductance at one end thereof, and electrically connected to a later mentioned third inductance at the other end thereof, and (d13) a third inductance electrically connected to the second inductance at one end thereof, and grounded at the other end thereof, the second switch is comprised of a switching device which short-circuits the third inductance to a ground when a radio signal is to be transmitted.
The second switch may be comprised of a GaAs semiconductor element switch.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, the controller switches the first switch when a radio signal is to be transmitted or received. The controller electrically connects the radio signal receiver to the adjuster when a radio signal is to be received, to thereby vary a circuit structure of the adjuster, and thus, match impedance between the antenna and the radio signal receiver in radio signal receiving band. The controller electrically connects the radio signal transmitter to the adjuster when a radio signal is to be transmitted, to thereby match impedance between the antenna and the radio signal transmitter in radio signal transmitting band.
Thus, the present invention makes it possible to change impedance matching between when a radio signal is to be transmitted and when a radio signal is to be received. As a result, impedance matching in a portable radio signal transceiver can be optimized, which ensures improvement in both transmission loss and antenna characteristic.
That is, since the present invention makes it possible to carry out impedance matching separately in a radio signal transmitting band and in a radio signal receiving band, transmission loss between an antenna and internal circuits can be reduced, thereby enhancing performances of a portable radio signal transceiver.
In addition, since impedance matching between input impedance of an antenna and characteristic impedance of circuits can be optimized, it would be possible to improve output-load characteristic in a radio signal transmitting circuit, and also possible to prevent degradation in a noise figure (NF) of a radio signal receiving circuit. As a result, it is also possible to prevent degradation in sensitivity in a radio signal receiving circuit, ensuring stability in performances of a portable radio signal transceiver.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.